Effects of morphine, levonantradol, and N-methyllevonantradol on shock intensity discrimination

The effects of morphine, levonantradol, and N-methyllevonantradol administered to squirrel monkeys by intramuscular injection were examined on an electric shock discrimination test. Two discrete responses were used to measure discrimination of the presence or absence of a shock at two different intensities (0.45 and 0.15 mA). The percentage of responses that were correct, the response latency, and the response probability in the presence or absence of shock were determined. Morphine (0.3-1.0 mg/kg) decreased the percentage of correct responses in the presence of both shock intensities, although effects at the higher intensity were slight. Levonantradol (0.017-0.03 mg/kg) and N-methyllevonantradol (0.01-0.017 mg/kg) only decreased the percentage of correct responses at the lower shock intensity, Neither morphine, levonantradol, nor N-methyllevonantradol consistently decreased the percentage of correct responses in the absence of shock. Morphine, levonantradol, and N-methyllevonantradol also produced dose-related increases in response latency both in the presence and absence of shock; increases in response latency were greater in the absence of shock than in the presence of shock.